Driving slower = better gas mileage. Fact or myth?

[KIAman]

RideFree brings up a good point. The value of getting better mileage is not necessarily about money rather consumption and pollution.

Also, people don't seem to understand that going from a 30MPG to 50MPG car is not nearly as money saving as going from 15MPG to 20MPG.

For all you truck buyers that think "oh that truck is 2MPG better than the other truck, no big deal" do not be fooled. It is a huge deal.

 

[aj654987]

Originally posted by: KIAman
To all the people who keep throwing in that driving slower takes longer to reach your destination... the metrics of saving gas is in MILES PER GALLON. If your MPG goes up slower, then it is irrelevant that it took longer to get there, you WILL be saving gas.

Also, read the link that I made earlier, it has a great description of gas consumption based on speed.

Snippet

In effect the efficiency of the engine is improving. It uses a fixed amount of fuel to power itself and the accessories, and a variable amount of fuel depending on the power required to keep the car going at a given speed. So in terms of fuel used per mile, the faster the car goes, the better use we make of that fixed amount of fuel required.

This trend continues to a point. Eventually, that road load curve catches up with us. Once the speed gets up into the 40 mph range each 1 mph increase in speed represents a significant increase in power required. Eventually, the power required increases more than the efficiency of the engine improves. At this point the mileage starts dropping. Let's plug some speeds into our equation and see how a 1 mph increase from 2 to 3 mph compares with a 1 mph increase from 50 to 51 mph. To make things easy we'll assume a, b and c are all equal to 1.

Speed

Equation

3 mph | 3+3^2+3^3 | 39
2 mph | 2+2^2+2^3 | 14
Power Increase = 25

51 mph | 51+51^2+51^3 | 135,303
50 mph | 50+50^2+50^3 | 127,550
Power Increase = 7,753

You can see that the increase in power required to go from 50 to 51 mph is much greater than to go from 2 to 3 mph.

So, for most cars, the "sweet spot" on the speedometer is in the range of 40-60 mph. Cars with a higher road load will reach the sweet spot at a lower speed. Some of the main factors that determine the road load of the car are:

* Coefficient of drag. This is an indicator of how aerodynamic a car is due only to its shape. The most aerodynamic cars today have a drag coefficient that is about half that of some pickups and SUVs.
* Frontal area. This depends mostly on the size of the car. Big SUVs have more than double the frontal area of some small cars.
* Weight. This affects the amount of drag the tires put on the car. Big SUVs can weigh two to three times what the smallest cars weigh.

In general, smaller, lighter, more aerodynamic cars will get their best mileage at higher speeds. Bigger, heavier, less aerodynamic vehicles will get their best mileage at lower speeds.

If you drive your car in the "sweet spot" you will get the best possible mileage for that car. If you go faster or slower, the mileage will get worse, but the closer you drive to the sweet spot the better mileage you will get.

Good source. The point is bold is key. It doesnt matter whether or not the air resistance is linear. Even if it was linear, the drag force is still increasing as the speed increases. The faster the car goes, the more drag force it has. While the engine efficiency increases at higher speeds it also has more drag force to over come, eventually there will be a point where the increased efficiency versus increased drag force and so there will be a speed where optiumum fuel efficiency is reached.

If the engine efficiency increased proportionally with the drag increase, so that it exactly matched it, then higher speeds would have the same fuel economy as lower speeds. Either they could both be linear or nonlinear so as long as they were proportionate.


The above just shows that after a certain point, higher speeds will be less fuel effiecient by some amount. (you would need data on how much and if it is even significant difference from say 60 to 70mph) as well as the time saved from travelling faster. I am not an advocate of lowering highway speeds. During rush hour, top speeds usually cant be reached any way due to congestion.

 

[aj654987]

Originally posted by: KIAman
RideFree brings up a good point. The value of getting better mileage is not necessarily about money rather consumption and pollution.

Also, people don't seem to understand that going from a 30MPG to 50MPG car is not nearly as money saving as going from 15MPG to 20MPG.

For all you truck buyers that think "oh that truck is 2MPG better than the other truck, no big deal" do not be fooled. It is a huge deal.

15 to 20 is 33% more efficient and 30 to 50 is 67% more efficient.


If you drove 200 miles a week, at $4 a gallon

15mpg $53.33 a week
20mpg $40.00 a week

$13.33 savings a week

30mpg $26.66 a week
50mpg $16.00 a week

10.66 savings a week



It comes out pretty close to the same savings.

 

[RideFree]

Originally posted by: RideFree
Diminishing returns, anyone?

Originally posted by: aj654987
...eventually there will be a point where the increased efficiency versus increased drag force and so there will be a speed where optimum fuel efficiency is reached.

Exactly the definition of the Point of Diminishing Returns as I recall from an ancient Calculus course.

 

[montag451]

Speed/revs definitely makes a difference to the economy.

When I first get my yellow beast, I floored it for a tank, then next tank kept it to <2500revs.
When pedal touch metal, I got about 520miles out of an 80 litre tank.
When treated like a delicate butterfly, 690miles.
That is a big difference and was measured over long distances.

 

[DrPizza]

I'd love to have a device that measures the exact rate at which fuel is being consumed in my vehicle so I'd know once and for all what it is the best speed for my vehicle. I'm also convinced that when I drive a grand caravan with a kayak on the roof, I get better mileage.

 

[KIAman]

Originally posted by: DrPizza
I'd love to have a device that measures the exact rate at which fuel is being consumed in my vehicle so I'd know once and for all what it is the best speed for my vehicle. I'm also convinced that when I drive a grand caravan with a kayak on the roof, I get better mileage.

A lot of cars offer this feature in the dash already. Of course, the science behind it does not truly offer 100% accuracy, rather an estimate based on the number of injector pulses of fuel into the combustion chambers.

As for the kayak, it probably decreases your drag coefficient enough that the added weight is negated and improves overall wind drag.

 

[KillerCharlie]

Originally posted by: KIAman

Originally posted by: DrPizza
As for the kayak, it probably decreases your drag coefficient

I highly, highly doubt a kayak will decrease drag...

 

[CycloWizard]

Originally posted by: KillerCharlie
I highly, highly doubt a kayak will decrease drag...

On most cars, probably not. On a Grand Caravan, however, I wouldn't be too surprised. Those things have the aerodynamic profile of a brick.

 

[Eeezee]

OP, this has been covered to death in off topic. I suggest you search around in there (although the search function is totally broken).

A few people found graphs. Peak fuel efficiency for most cars is in the 50-60 mph range. That's assuming you're not screwing around with stop lights of course; it pays off more to go even slower than 50 in that situation, since you're forced to stop at certain intervals (or at least drive the right speed so you hit every green light).

What you need to know is that air resistance is not linear; laminar flow stops at really low speeds. It's a good approximation to say that if you're driving, you are outside of the laminar flow (linear air resistance).

Since air friction is increasing as v^2 when you're driving a car, it's obvious that higher speed = more force required to accelerate. An analogy could be made to the speed of light here; consider the maximum velocity you can travel as being that where your engine can no longer overcome the air drag due to velocity^2. As you begin to approach this maximum velocity, your net acceleration is steadily decreasing even though you're putting in the same amount of force.

 

[CycloWizard]

Originally posted by: Eeezee
OP, this has been covered to death in off topic. I suggest you search around in there (although the search function is totally broken).

The discussions of this topic in OT and P&N is a joke. This is the only forum here that could actually pretend to tackle this subject with any amount of rigor.

A few people found graphs. Peak fuel efficiency for most cars is in the 50-60 mph range. That's assuming you're not screwing around with stop lights of course; it pays off more to go even slower than 50 in that situation, since you're forced to stop at certain intervals (or at least drive the right speed so you hit every green light).

What you need to know is that air resistance is not linear; laminar flow stops at really low speeds. It's a good approximation to say that if you're driving, you are outside of the laminar flow (linear air resistance).

Since air friction is increasing as v^2 when you're driving a car, it's obvious that higher speed = more force required to accelerate. An analogy could be made to the speed of light here; consider the maximum velocity you can travel as being that where your engine can no longer overcome the air drag due to velocity^2. As you begin to approach this maximum velocity, your net acceleration is steadily decreasing even though you're putting in the same amount of force.

That doesn't really make any sense. Any of it. The first paragraph I'm not even going to address, since there is so much per-car variability that such generalizations are meaningless. The second paragraph is similar in that there is an enormous amount of per-car variability. The third paragraph is inaccurate at best and has been addressed by myself previously in this thread.

 

[Comdrpopnfresh]

The reasoning is dumbed down. You obviously experience less drag going slower- which is a large portion of fuel consumption. An engine's sweet-spot for rpms might be an issue too. But one of the largest fuel-wasting things is acceleration. ICE's are pretty good at 100% load and/or steady rpms, but changing them leads to higher consumption. That's why the latest electric car designs using an ICE as a generator are so efficient- not because they run on electricity, but because this is a method to allow the engine to run at constant load regardless of driver habits, speed, and acceleration.

 

[NeoPTLD]

With an engine that makes power at exactly the same efficiency, calculation is easier, however the efficiency varies due to many factors.

Exponential increase in drag means that you will get less miles per wheel kWh

 

[CycloWizard]

Originally posted by: Comdrpopnfresh
The reasoning is dumbed down. You obviously experience less drag going slower- which is a large portion of fuel consumption.

Originally posted by: NeoPTLD
With an engine that makes power at exactly the same efficiency, calculation is easier, however the efficiency varies due to many factors.

Exponential increase in drag means that you will get less miles per wheel kWh

Seriously?

 

[Eeezee]

Originally posted by: CycloWizard

Originally posted by: Eeezee
OP, this has been covered to death in off topic. I suggest you search around in there (although the search function is totally broken).

The discussions of this topic in OT and P&N is a joke. This is the only forum here that could actually pretend to tackle this subject with any amount of rigor.

A few people found graphs. Peak fuel efficiency for most cars is in the 50-60 mph range. That's assuming you're not screwing around with stop lights of course; it pays off more to go even slower than 50 in that situation, since you're forced to stop at certain intervals (or at least drive the right speed so you hit every green light).

What you need to know is that air resistance is not linear; laminar flow stops at really low speeds. It's a good approximation to say that if you're driving, you are outside of the laminar flow (linear air resistance).

Since air friction is increasing as v^2 when you're driving a car, it's obvious that higher speed = more force required to accelerate. An analogy could be made to the speed of light here; consider the maximum velocity you can travel as being that where your engine can no longer overcome the air drag due to velocity^2. As you begin to approach this maximum velocity, your net acceleration is steadily decreasing even though you're putting in the same amount of force.

That doesn't really make any sense. Any of it. The first paragraph I'm not even going to address, since there is so much per-car variability that such generalizations are meaningless. The second paragraph is similar in that there is an enormous amount of per-car variability. The third paragraph is inaccurate at best and has been addressed by myself previously in this thread.

No, cars really do experience a v^2 drag when driving at any reasonable speed. The Reynolds number for even the most aerodynamic car is large even at low velocities. What about that doesn't make sense? Vehicles experience v^2 drag when not driving at a crawl, so you can expect the difference in mileage between 120mph and 100mph to be enormous.

If that doesn't make sense, why don't you point out some inconsistencies or use some reasoning?

What is inaccurate about the third paragraph? I thought it was a reasonable analogy; perhaps there doesn't exist a car engine that can get your car to maximum speed, but such a speed does exist, and as you approach it you require more force to maintain the same net acceleration. That's just physics; higher drag = more energy spent by the engine to maintain your current acceleration (either positive or 0 if you want to maintain your current velocity). What about it was inaccurate?

Also, you're being an asshole. Please tell me why none of that makes sense instead of simply stating that it doesn't without backing up your claim.

 

[CycloWizard]

Originally posted by: Eeezee
No, cars really do experience a v^2 drag when driving at any reasonable speed. The Reynolds number for even the most aerodynamic car is large even at low velocities. What about that doesn't make sense? Vehicles experience v^2 drag when not driving at a crawl, so you can expect the difference in mileage between 120mph and 100mph to be enormous.

If that doesn't make sense, why don't you point out some inconsistencies or use some reasoning?

What is inaccurate about the third paragraph? I thought it was a reasonable analogy; perhaps there doesn't exist a car engine that can get your car to maximum speed, but such a speed does exist, and as you approach it you require more force to maintain the same net acceleration. That's just physics; higher drag = more energy spent by the engine to maintain your current acceleration (either positive or 0 if you want to maintain your current velocity). What about it was inaccurate?

Also, you're being an asshole. Please tell me why none of that makes sense instead of simply stating that it doesn't without backing up your claim.

Why don't you read the thread before posting in it? All of this has already been addressed in this very thread, by me no less, though apparently not wanting to repeat myself ad nauseum makes me an asshole. But, since you're obviously both physically and intellectually lazy, I'll do the legwork for you and quote my previous statements.

about the Rayleigh equation for drag force

There are a few assumptions that go into that formulation that limit its applicability to this discussion. The Rayleigh equation gives the drag on a particle in an infinite medium. Obviously, this doesn't really hold up for a car, since there are always interactions between the bottom (and even sides) of the car and the road. Further, it only holds at asymptotically high Reynolds numbers (i.e. viscous effects are completely negligible and flow is fully turbulent), which is incorrect for nicely designed cars. A lot of car manufacturers have spent a great deal of time treating the bottom of the car to reduce drag. If it were as simple as applying this equation, wind tunnels would barely be used.

If you had ever seen a friction factor-Reynolds number plot (AKA Moody diagram), which are ubiquitous for anyone studying fluid mechanics, it is well known that these effects are very, very nonlinear and depend on a lot more than the velocity. And the Moody diagram is for flow through a pipe of uniform cross section, which is obviously much simpler than flow of a fluid around an irregularly shaped object like a car.

So, if you have any data on the local Reynolds numbers observed during driving, please share them. Otherwise, please stop asserting that drag always scales as the square of the velocity, since that is obviously not the case.

 

[Eeezee]

I see where you're confused now; you just completely missed the point.

So long as there is drag, traveling faster will increase the force required to maintain current acceleration. Since the relationship at these speeds is non-linear, you will expend more energy in reaching your destination despite getting there faster. That's the point. It could be a dependence of v^1.0001 and that would still be true, but v^2 is a fine approximation. Next you'll be nitpicking me on the fact that c isn't exactly 3*10^8 m/s.

Yes, you've brought up that the relationship is not purely v^2, but it is never linear (unless the velocity is very low of course), and that is the point. Why are you arguing? The fact of the matter is that peak mpg for a car is never above 70mph, and that's what the OP was asking about. He wasn't asking for an analysis for a particular vehicle, he was contesting a true claim that peak mpg occurs at lower velocities.

Also, I did read the entire thread; I was confused as to why you were arguing, but it turned out you were just arguing for the sake of arguing. It's great that you're able to argue with people who agree with you, although it really just indicates that you have nothing better to do... and it makes you an asshole, so there's that.

By the way, you never addressed the third paragraph that you claimed didn't make sense. There exists a maximum speed at which any object can travel in velocity-dependent drag, even laminar flow. If you observe some flaw in that statement, then please enlighten us. I doubt you'll be able to, and that, in particular, just makes you an asshole; you nitpicked without providing any reasoning. You're a troll, and that becomes more apparent each time that you reply.

 

[CycloWizard]

Originally posted by: Eeezee
By the way, you never addressed the third paragraph that you claimed didn't make sense. There exists a maximum speed at which any object can travel in velocity-dependent drag, even laminar flow. If you observe some flaw in that statement, then please enlighten us. I doubt you'll be able to, and that, in particular, just makes you an asshole; you nitpicked without providing any reasoning. You're a troll, and that becomes more apparent each time that you reply.

Let's get back to that paragraph, shall we?

Since air friction is increasing as v^2 when you're driving a car, it's obvious that higher speed = more force required to accelerate. An analogy could be made to the speed of light here; consider the maximum velocity you can travel as being that where your engine can no longer overcome the air drag due to velocity^2. As you begin to approach this maximum velocity, your net acceleration is steadily decreasing even though you're putting in the same amount of force.

Do you really think that cars are operating in this regime - that we drive that close to the terminal velocity? Of course not, but this is the only regime you're considering. You keep saying "It's obvious that [more work is needed to accelerate at higher speeds]." I'm not sure if you were ever formally introduced to debate or logic, but this is known as begging the question. "It's obvious that..." is exactly where you're wrong because the assertion that follows isn't even correct. That is completely obvious from the Moody diagram that I posted. If I start from 0 mph and accelerate, my vehicle experiences all sorts of transitions in flow regimes before I reach terminal velocity (assuming my engine could even reach that velocity, which is unlikely). Of course, as you approach the terminal velocity, you're correct. What you don't seem to get is that most driving (indeed, all unless you happen to drive a racecar) happens far from that limit.

edited to remove responses to personal attacks.

 

[BassBomb]

Garage had quite agood topic on this

 

[Wellsoul2]

Originally posted by: Cogman
as far as I understood it. Driving in the highest gear with about the lowest gas in would give you the best gas milage (about). For most automatics that comes in somewhere between 50-60 mph if I remember correctly.

Driving in a low gear but high RPM obviously doesn't give you good gas milage because the lower gear is less efficient. Now when you start driving faster, IE over 75 that is when you start seeing your gas milage drop off, somewhat dramatically.

It would seem logical that the speed at which you shift into the highest gear..or 5 MPH
over would get the best gas mileage.

 

[tomcat2200]

As I recall, the original speed reductions on the highways was to mitigate fatalities, not for saving fuel.

A huge factor forgotten in most of these conjectures is the cost to slow down ground freight transportation. This has a significant economic impact on our infrastructure. Most freight, even if tagged as next day air, still goes by ground, if the trucks can reach their destination within the allotted time. This translates to roughly 1300 miles with 55mph speed limits.

It may be a bit of a complex subject for politicians or consumers to calculate the impact, but there is an effect none the less. As I recall, the speed limit reductions, were followed by a recession in the day. It was bad enough that fuel costs were driving up prices, as we see today as well. Consider the nation wide cost of time. As they used to say "time is money". In the context of the infrastructure, it is actually true.

 

[CycloWizard]

Originally posted by: tomcat2200
As I recall, the original speed reductions on the highways was to mitigate fatalities, not for saving fuel.

A huge factor forgotten in most of these conjectures is the cost to slow down ground freight transportation. This has a significant economic impact on our infrastructure. Most freight, even if tagged as next day air, still goes by ground, if the trucks can reach their destination within the allotted time. This translates to roughly 1300 miles with 55mph speed limits.

It may be a bit of a complex subject for politicians or consumers to calculate the impact, but there is an effect none the less. As I recall, the speed limit reductions, were followed by a recession in the day. It was bad enough that fuel costs were driving up prices, as we see today as well. Consider the nation wide cost of time. As they used to say "time is money". In the context of the infrastructure, it is actually true.

Railroads FTW. That would solve a lot of this nation's problems:
1. Traffic congestion.
2. Accelerated road wear, and
3. Energy consumption.

 

[Biftheunderstudy]

Originally posted by: CycloWizard

Railroads FTW. That would solve a lot of this nation's problems:
1. Traffic congestion.
2. Accelerated road wear, and
3. Energy consumption.

Maglev anyone?
Starting to feel like alpha centauri....

 

[PlasmaBomb]

They are thinking about it. The costs involved are huge though...

 

[designerfx]

Remember that MPG is affected by the gear ratio in addition to forces such as drag. Also remember that cars RPMs will come into the equation separate of gear ratio. If you're driving and your car is only running at 2000 rpms (and think of how much gas is used for each ignition of the spark plugs), but you're running it for an extra 10 minutes versus 6000 rpms for 2 minutes, there is a point at which driving slow is actually pretty moot. You're looking at a very large amount of effort in driving slow for a minimal increase in mileage at an exponential decrease in safety for you and those around you.

So short answer for question = fact, however long answer = there's more to it.

Also, driving slow is hazardous to other drivers on the road. Thankfully people in many states around the world give tickets for that now.

If people are so worried about mileage, get a compact car/small engine vehicle/diesel car. Learn to drive a stickshift. Never get a hybrid (prius, civic, anything), they are 100% marketing hype.

Also, railroads sound good but you know, they have limits. I'm all for mass transportation but only if it is maglev/more efficient methods. Oldschool 1950's era/older "railroads" are horrible.